Low temperature charging of li-ion cells

ABSTRACT

A battery cell charging system, including a charger and a controller, for low-temperature (below about zero degrees Celsius) charging a lithium ion battery cell, the battery cell charging system includes: a circuit for charging the battery cell using an adjustable voltage charging-profile to apply a charging voltage and a charging current to the battery cell wherein the adjustable voltage charging-profile having: a non-low-temperature charging stage for charging the battery cell using a charging profile adapted for battery cell temperatures above about zero degrees Celsius; and a low-temperature charging stage with a variable low-temperature stage charging current that decreases responsive to a battery cell temperature falling below zero degrees Celsius.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/226,636, filed 17 Jul. 2009, the contents of which are expresslyincorporated in their entirety for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to rechargeable lithium-ion-typechemistry batteries, and more specifically to temperature dependentcharging of automotive Li-ion battery packs, particularly to “lowtemperature” charging.

The defense, automotive and aerospace applications include operation atextreme temperature ranges beyond those used in the consumerelectronics. For example, in an application for an electric vehicle, theuser will expect that the automobile will be operational at lowtemperature. This operation includes an expectation for being able torecharge the batteries used in the vehicle. Beyond meeting consumerexpectations, it is an important safety consideration to enable thevehicle to operate in lower temperature ranges. For purposes of thisapplication, low temperature is defined at temperatures less than aboutzero degrees Celsius.

Lithium ion batteries are common in consumer electronics. They are oneof the most popular types of battery for portable electronics, with oneof the best energy-to-weight ratios, no memory effect, and a slow lossof charge when not in use. In addition to uses for consumer electronics,lithium-ion batteries are growing in popularity for defense, automotive,and aerospace applications due to their high energy and power density.However, certain kinds of treatment may cause Li-ion batteries to failin potentially dangerous ways.

One of the advantages of use of a Li-ion chemistry is that batteriesmade using this technology are rechargeable. Traditional charging isdone with a two-step charge algorithm: (i) constant current (CC), and(ii) constant voltage (CV). In electric vehicles (EVs), the first stepcould be constant power (CP).

Step 1: Apply charging current limit until the volt limit per cell isreached.

Step 2: Apply maximum volt per cell limit until the current declinesbelow a predetermined level (often C/20 but sometimes C/5 or C/10 orother value).

The charge time is approximately 1-5 hours depending upon application.Generally cell phone type of batteries can be charged at 1 C, laptoptypes 0.8 C. The charging typically is halted when the current goesbelow C/10. Some fast chargers stop before step 2 starts and claim thebattery is ready at about a 70% charge. (As used herein, “C” is a ratedcurrent that discharges the battery in one hour.)

Generally for consumer electronics, lithium-ion is charged withapproximate 4.2±0.05 V/cell. Heavy automotive, industrial, and militaryapplication may use lower voltages to extend battery life. Manyprotection circuits cut off when either >4.3 V or 90° C. is reached.

Battery chargers for charging lithium-ion-type batteries are known inthe art. As is known in the art, such lithium ion batteries requireconstant current (CC) and constant voltage (CV) charging. In particular,initially such lithium ion batteries are charged with a constantcurrent. In the constant current mode, the charging voltage is typicallyset to a maximum level recommended by the Li-ion cell manufacturer basedon safety considerations, typically 4.2V per cell. The charging currentis a factor of design level, based on the cell capability, charge time,needs and cost. Once the battery cell voltage rises sufficiently, thecharging current drops below the initial charge current level. Inparticular, when the battery cell voltage Vb approaches the chargingvoltage Vc, the charging current tapers according to the formula:I=(Vc−Vb)/Rs, where I=the charging current, Vc=the charging voltage,Vb=the battery cell open circuit voltage and Rs=the resistance of thecharging circuit including the contact resistance and the internalresistance of the battery cell. As such, during the last portion of thecharging cycle, typically about the last ⅓, the battery cell is chargedat a reduced charging current, which means it takes more time to fullycharge the battery cell.

The closed-circuit voltage represents the voltage of the battery cellplus the voltage drops in the circuit as a result of resistance in thebattery circuit, such as the battery terminals and the internalresistance of the battery cell. By subtracting the closed-circuitvoltage from the open-circuit voltage, the voltage drop across thebattery resistance circuit elements can be determined.

Various known battery chargers use this voltage drop to drive thebattery charging voltage during a constant current mode in order toincrease the Ampere-hour (Ah) applied to the battery cell during aconstant current mode. By increasing the Ah applied to the battery cellduring a constant current mode, the battery cell is charged much faster.

It is known to include lithium ion battery charger circuits thatcompensate for the voltage rise in the battery circuit in order toincrease the charging current and thus decrease the charging time for alithium ion battery. The compensation circuit can be based on an assumedinitial voltage drop across the various resistance elements in thecircuit and compensates for this voltage drop to maintain apredetermined charging current during a constant current charging mode.Unfortunately, the resistance of the various resistance elements changeover time due to various factors including oxidation of the externalbattery contacts used to connect the battery cell to the batterycharger. Accordingly, in time, the charging time of the battery cellincreases.

Most lithium-ion battery manufacturers produce specifications for theirbatteries that do not include a provision for charging at lowtemperatures (that is, charging is stopped when a cell temperature fallsbelow a certain threshold (typically zero degrees C.)). Traditionallycharging a lithium-ion battery at less than zero degrees C. at normalrates and voltages causes lithium plating on the anode which degradescycle life. This hasn't been an in issue in consumer electronicapplications. However, in automotive applications this is unacceptablesince the automobile is a safety critical and convenient productoperating in the extremes. Also, the battery packs used in electricvehicles are quite sophisticated and typically have a high cost toreplace. Manufacturers and consumers both desire to preserve the lifecycle of these battery packs.

What is needed is a battery charger and battery charging process forextending a charging profile for a lithium-ion battery pack to includelow temperature charging.

BRIEF SUMMARY OF THE INVENTION

Disclosed is a battery charger and battery charging process forextending a charging profile for a lithium-ion battery pack to includelow temperature charging. The preferred embodiments do not includetemperature independent current and voltage steps.

A battery cell charging system, including a charger and a controller,for low-temperature (below about zero degrees Celsius) charging alithium ion battery cell, the battery cell charging system includes: acircuit for charging the battery cell using an adjustable voltagecharging-profile to apply a charging voltage and a charging current tothe battery cell wherein the adjustable voltage charging-profile having:a non-low-temperature charging stage for charging the battery cell usinga charging profile adapted for battery cell temperatures above aboutzero degrees Celsius; and a low-temperature charging stage with avariable low-temperature stage charging current that decreasesresponsive to a battery cell temperature falling below zero degreesCelsius.

A battery cell charging system for low-temperature (below about zerodegrees Celsius) charging a lithium ion battery cell, the battery cellcharging system including: a circuit for charging the battery cell usinga variable charge-profile to apply a charging voltage and a chargingcurrent to the battery cell wherein the charge-profile is determinedresponsive to an imaginary R_(Bad) resistance included in series with anDCR resistance of a DC model for the battery cell wherein the R_(Bad) isa function including a component responsive to a battery celltemperature less than about zero degrees Celsius such that V_(cell) isabout equal to a maximum battery cell voltage minus charging currenttimes R_(Bad).

A battery cell charging method for low-temperature (below about zerodegrees Celsius) charging a lithium ion battery cell, the battery cellcharging method including: (a) applying, to the battery cell when atemperature of the battery cell is above about zero degrees Celsius, anon-low-temperature charging profile; and (b) applying, to the batterycell when a temperature of the battery cell is below about zero degreesCelsius, a variable low-temperature stage charging current thatdecreases responsive to a battery cell temperature falling below zerodegrees Celsius.

A battery cell charging method for low-temperature (below about zerodegrees Celsius) charging a lithium ion battery cell, the battery cellcharging method including: (a) applying, responsive to a charge-profile,a varying charging voltage and a varying charging current to the batterycell wherein the charge-profile is determined responsive to an imaginaryR_(Bad) resistance included in series with an DCR resistance of a DCmodel for the battery cell wherein the R_(Bad) is a function including acomponent responsive to a battery cell temperature less than about zerodegrees Celsius such that:

V _(Cell)≈TARGET_VOLTAGE−iBat*R _(Bad).   (1)

As noted above, typically, a li-ion battery cell is charged using a twostep CC-CV (constant-current constant voltage) algorithm. For highenergy cells, CC is typically 1 C or less and CV is between 4.1V and4.2V.

When chargers and charging methods of the prior art are used for fastcharging, cycle life degradation that occurs when the cell is charged athigh rates (above 1 C for typical consumer 18650 cells and other energycells) and at high voltages as the prior art inventions mentionedpreviously. By modifying the apparatus and process to include a four (ormore) steps cycle life degradation even at 2 C charge rates is reduced.The multistate charge-profile as follows was employed with minimal cycledegradation: 2 C, 4.0V (hold until 0.7 A), 0.7 A, 4.2V (CC-CV-CC-CV).While an n-step adjustable charge profile (n=4) is described, n may haveother values. Prior art systems use a two-stage profile having aconstant current stage 1 applied until cell voltage limit is reached,and then a constant voltage stage 2 at max cell voltage is applied untila “full” charge is reached. The present embodiments insert one or moreintermediate charging stages that improve charging rate withoutdegrading battery performance. The intermediate stages include one ormore of constant voltage stages (iBat is decreasing) or constant currentas battery charge voltage increases or an intermediate stage wherehigher current is used due to the cell chemistry or the temperature thathappens to occur at some point during charge.

In a particular implementation, the battery may be modeled to decreasecharging current as battery voltage increases. The model may include anR_(Bad) that allows improved charging rates without the negativeconsequences. The model would represent R_(Bad) as a function of thevarious physical parameters that can negatively affect cycle life. Forexample, R_(Bad) could partially represent the polarization of thenegative electrode (R_(anode)). As R_(anode) increases, the likelihoodof lithium plating on the anode is increased, thereby leading tocapacity fade in the battery. So from a modeling perspective, theparameters that affect R_(anode,) such as temperature (includinglow-temperature), cell age and state of charge can appropriately beadjusted in the charge algorithm to prevent cell damage. There areseveral examples of these physical, cell level parameters that affectR_(Bad) such electrolyte type, electrode design, and anode material. Allof these parameters can be experimentally determined and thereforeproperly modeled to show their effect on a R_(Bad) value.

One source of capacity degradation consequent to low-temperaturecharging results from side reactions in the negative electrode (i.e.,lithium plating). Reducing the charge currents when battery celltemperature is low minimizes the side reactions and thereby reducesdegradation due to low-temperature charging.

Use of the low-temperature charging profiles makes electric-powereddevices more readily available. In conventional systems, either a userwould forego charging at low temperatures or the manufacturer of thedevice would include more complex and costly systems to enable charging,such as battery heaters. The low temperature charging thus permitsgreater availability or reduced cost, or both, depending upon previoussolutions.

Other advantages of the present invention will be seen by review of thepresent disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart of a simplified multistage (4 stage) fast chargeprofile for a battery charger;

FIG. 2 is a schematic diagram of a DC model of a lithium-ion batterycell including an imaginary R_(Bad) element;

FIG. 3 is a variable fast charge profile for a battery chargerresponsive to the R_(Bad) element shown in FIG. 2;

FIG. 4 is a representative charging system;

FIG. 5 is a control diagram for the charging system shown in FIG. 4.

FIG. 6 is an illustration of a low-temperature charging profile for usewith the charging system shown in FIG. 4; and

FIG. 7 is a variable low-temperature charge profile for a batterycharger responsive to the R_(Bad) element shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a system and method for alow-temperature charger, particularly for lithium-ion battery cellswhile reducing/eliminating the impact of low-temperature charging oncycle life. The following description is presented to enable one ofordinary skill in the art to make and use the invention and is providedin the context of a patent application and its requirements. Variousmodifications to the preferred embodiment and the generic principles andfeatures described herein will be readily apparent to those skilled inthe art. Thus, the present invention is not intended to be limited tothe embodiment shown but is to be accorded the widest scope consistentwith the principles and features described herein.

FIG. 1 is a chart of a simplified multistage fast charge profile for abattery charger. The preferred embodiment for the fast charge profileincludes at least four stages: a CC (constant current) first stage, aconstant voltage (CV) second stage; a CC third stage, and a CV fourthstage.

The first stage includes CC at a level greater than the typical priorart value of 1 C. The first stage voltage varies from an initial valueless than the cell target voltage to about 4.0V to about 4.05V. Theseare representative values but the actual value depends upon an impedanceof the battery cell being charged. A lower impedance battery cell isable to be charged at higher voltages without degradation. The actualvalue also depends upon the maximum cell voltage, so the first stagetarget voltage may be a percentage of the maximum voltage, for example80% SOC.

The second stage includes CV at the charging voltage level reached atthe end of the first stage (e.g., 4.0-4.05 volts). During the secondstage, the charging current declines from the first stage value of(preferably) 2 C to about 0.7 C.

The third stage includes CC at the second stage level while the chargingvoltage of the third stage increases to about the cell target voltage.

The fourth stage includes CV at about the cell target voltage while thecharging current of the fourth stage declines. When the charging currentof the fourth stage declines below a predetermined level, then thecharging cycle is complete. These values are determined by application,typically C/10, sometimes C/5 or C/20. In an EV, C/70 could be thecharging level.

FIG. 2 is a schematic diagram of a DC model of a lithium-ion batterycell including an imaginary R_(Bad) element. The use of the R_(Bad)element permits a different embodiment to produce a continually variablevoltage charging profile as compared to FIG. 1 that produces a similarperformance but may result in improved cycle life by lowing a voltagepoint where a final voltage taper begins. R_(Bad), by taking intoaccount battery age or a high impedance state, improves cycle lifebecause lithium plating occurrence is directly related to battery ageand the high impedance state.

In general, as impedance increases (with age, cycles, low temperatures),the voltage target can decrease. R_(Bad) is introduced into a DC batterycell model to describe a charging method that defines an adjustablevoltage level and SOC (state-of-charge) point at which taper begins. Themodels for the battery (e.g., R_(Bad) and V_(Negative) _(—) _(Anode))are ways to mathematically determine, and to use in control systems,when excessively high charging currents at those cell conditions maycause irreversible capacity loss.

FIG. 3 is a variable fast charge profile for a battery chargerresponsive to the R_(Bad) element shown in FIG. 2. Initially the profileincludes a CC (or alternatively CP) at maximum current (which in somecases can be 5 C or greater) and an increasing charging voltageV_(cell). V_(cell) is equal to the target voltage minus iBat timesR_(Bad). The R_(Bad) element controls the tapering of the chargingcurrent and the charging voltage because iBat decreases as V_(Bad) goesdown and R_(Bad) goes up as SOC increases. These relationships are shownin FIG. 3.

R_(Bad) of the preferred embodiment is based on a percentage of DCResistance (DCR). At 25 degrees C., DCR is R_(Nominal), but as thepreferred embodiments address charging at other than nominaltemperature, the value DCR is used. As an example, R_(Bad)=k1×DCR+f(SOC)where f(SOC) is a function/lookup table using SOC as an input. Forexample, k1 is between 0 and 1 (typically around 0.1) and f(SOC) couldbe k2×SOC where k2 is typically around 0.001/SOC%. More generically,R_(Bad) may be described as a function g(SOC, Temperature, DCR) andcould also be a function of age, although typically as a battery agesits nominal impedance goes up. DCR varies based on battery celltemperature, age and SOC. DCR may be determined in different ways, suchas, for example, by look-up table or calculated in real-time. Generally,R_(Bad) is directly related to DCR and SOC and inversely related totemperature. With this approach targetV=TARGET_VOLTAGE−V_(Bad).TARGET_VOLTAGE is the final voltage that one wants to achieve at the endof charge for each cell in the battery pack, typically 4.2V, and V_(Bad)is iBat times R_(Bad). Table I provides representative values in theexample of an electric vehicle having a 4.18V target voltage and a 150Ah capacity battery.

TABLE I DCR R_(Bad) SOC (mOhm) (mOhm) OCV iBat vBad 0 18 1.8 3 60.6 0.2010 12 1.21 3.4 60.6 .073 20 9 0.92 3.5 70.6 0.065 30 7.5 0.78 3.6 72.50.056 40 6 0.64 3.7 75.3 0.048 50 4.5 0.5 3.8 80.0 0.040 60 4.5 0.513.85 68.8 0.036 70 4.5 0.52 3.9 59.8 0.031 80 4.5 0.53 4 39.8 0.021 904.5 0.54 4.1 19.0 0.010 100 4.5 0.65 4.18 0.0 0.000

Another implementation method is based on the estimated anode voltagereferenced to Li+. It believed that, in present li-ion technology, fastcharging damages the anode when its voltage drops towards 0V referencedto Li+. So R_(Bad) could also be a function of V_(Negative) _(—)_(Electrode) _(—) _(Loaded), where V_(Negative) _(—) _(Electrode) _(—)_(Loaded)=V_(Negative) _(—) _(Electrode) _(—)_(ToLi)−current×r_(Negative) _(—) _(Electrode).

FIG. 4 is a preferred embodiment for a charging system 400, such as maybe used in an electric vehicle. System 400 includes a battery 405, acharger 410 coupled to battery 405 and a battery management system (BMS)415 and a battery data acquisition and monitoring subsystem 420. Acommunication bus 425 couples subsystem 420 to BMS 415 and acommunication bus 430 couples BMS 415 to charger 410. A communicationbus 435 couples battery data from battery 405 to subsystem 420.

Battery 405 is shown as a series-connected group of battery cells,however the arrangement of cells may be a combination of parallel/seriesconnected cells of many different arrangements. Charger 410 of thepreferred embodiment provides the charging current applied to battery405. BMS 415 controls the charging current according to a profileestablished by the embodiments of the present invention. Subsystem 420acquires the desired data as described herein regarding battery 405. Forexample, voltage, SOC, temperature, and other applicable data used byBMS 415. In some embodiments, subsystem 420 may be part of BMS 415 andBMS 415 may be part of charger 410. One or more of charger 410, BMS 415,and subsystem 420 control a switch 440.

FIG. 5 is a control diagram 500 for the charging system shown in FIG. 5.Diagram 500 describes a typical control system as may be used forcharging lithium ion cells. A target voltage 505 and a maximum cellvoltage 510 are subtracted and used by a controller 515 to produce acharging current 520. In prior art systems, current 520 is constant orcompensates for an internal resistance (IR) drop of battery 405. Asdescribed above, the preferred embodiments of the present inventiondescribe an adjusting charging current. In broad terms, as the voltageof the battery increases, the charging current decreases in aparticularly controlled manner to provide for fast charging withoutdegrading battery performance.

FIG. 6 is an illustration of a low-temperature charging profile for usewith the charging system shown in FIG. 4. In contrast to conventionalLi-ion battery chargers that use a conventional two-step charge sequencethat is stopped when battery cell temperature falls below thelow-temperature threshold (typically zero degrees Celsius), there are notemperature independent current and voltage steps. As the battery celltemperature goes down the charge current and voltage is reduced. FIG. 6relates charge current and charge voltage limit to temperature accordingto one embodiment of the present invention. As illustrated, at thetypical low temperature threshold, the embodiment of FIG. 6 begins todecrease iBatLimit and vBatLimit. Limits shown in FIG. 6 may,additionally and optionally, be based on an age of the battery cellsince the Li-ion battery cell's impedance increases with age.

FIG. 7 is a variable low-temperature charge profile for a batterycharger responsive to the R_(Bad) element shown in FIG. 2. In thisembodiment, R_(Bad) is modeled to include estimations of low-temperatureconditions of the battery cell. FIG. 7 also illustrates a differencebetween conventional charging and low-temperature charging.

The system above has been described in the preferred embodiment of anembedded automobile (EV) electric charging system. The system, method,and computer program product described in this application may, ofcourse, be embodied in hardware; e.g., within or coupled to a CentralProcessing Unit (“CPU”), microprocessor, microcontroller, System on Chip(“SOC”), or any other programmable device. Additionally, the system,method, and computer program product, may be embodied in software (e.g.,computer readable code, program code, instructions and/or data disposedin any form, such as source, object or machine language) disposed, forexample, in a computer usable (e.g., readable) medium configured tostore the software. Such software enables the function, fabrication,modeling, simulation, description and/or testing of the apparatus andprocesses described herein. For example, this can be accomplishedthrough the use of general programming languages (e.g., C, C++), GDSIIdatabases, hardware description languages (HDL) including Verilog HDL,VHDL, AHDL (Altera HDL) and so on, or other available programs,databases, nanoprocessing, and/or circuit (i.e., schematic) capturetools. Such software can be disposed in any known computer usable mediumincluding semiconductor (Flash, or EEPROM, ROM), magnetic disk, opticaldisc (e.g., CD-ROM, DVD-ROM, etc.) and as a computer data signalembodied in a computer usable (e.g., readable) transmission medium(e.g., carrier wave or any other medium including digital, optical, oranalog-based medium). As such, the software can be transmitted overcommunication networks including the Internet and intranets. A system,method, computer program product, and propagated signal embodied insoftware may be included in a semiconductor intellectual property core(e.g., embodied in HDL) and transformed to hardware in the production ofintegrated circuits. Additionally, a system, method, computer programproduct, and propagated signal as described herein may be embodied as acombination of hardware and software.

One of the preferred implementations of the present invention is as aroutine in an operating system made up of programming steps orinstructions resident in a memory of a computing system as well known,during computer operations. Until required by the computer system, theprogram instructions may be stored in another readable medium, e.g. in adisk drive, or in a removable memory, such as an optical disk for use ina CD ROM computer input or other portable memory system for use intransferring the programming steps into an embedded memory used in thecharger. Further, the program instructions may be stored in the memoryof another computer prior to use in the system of the present inventionand transmitted over a LAN or a WAN, such as the Internet, when requiredby the user of the present invention. One skilled in the art shouldappreciate that the processes controlling the present invention arecapable of being distributed in the form of computer readable media in avariety of forms.

Any suitable programming language can be used to implement the routinesof the present invention including C, C++, Java, assembly language, etc.Different programming techniques can be employed such as procedural orobject oriented. The routines can execute on a single processing deviceor multiple processors. Although the steps, operations or computationsmay be presented in a specific order, this order may be changed indifferent embodiments. In some embodiments, multiple steps shown assequential in this specification can be performed at the same time. Thesequence of operations described herein can be interrupted, suspended,or otherwise controlled by another process, such as an operating system,kernel, and the like. The routines can operate in an operating systemenvironment or as stand-alone routines occupying all, or a substantialpart, of the system processing.

In the description herein, numerous specific details are provided, suchas examples of components and/or methods, to provide a thoroughunderstanding of embodiments of the present invention. One skilled inthe relevant art will recognize, however, that an embodiment of theinvention can be practiced without one or more of the specific details,or with other apparatus, systems, assemblies, methods, components,materials, parts, and/or the like. In other instances, well-knownstructures, materials, or operations are not specifically shown ordescribed in detail to avoid obscuring aspects of embodiments of thepresent invention.

A “computer-readable medium” for purposes of embodiments of the presentinvention may be any medium that can contain, store, communicate,propagate, or transport the program for use by or in connection with theinstruction execution system, apparatus, system or device. The computerreadable medium can be, by way of example only but not by limitation, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, system, device, propagation medium, orcomputer memory.

A “processor” or “process” includes any human, hardware and/or softwaresystem, mechanism or component that processes data, signals or otherinformation. A processor can include a system with a general-purposecentral processing unit, multiple processing units, dedicated circuitryfor achieving functionality, or other systems. Processing need not belimited to a geographic location, or have temporal limitations. Forexample, a processor can perform its functions in “real time,”“offline,” in a “batch mode,” etc. Portions of processing can beperformed at different times and at different locations, by different(or the same) processing systems.

Reference throughout this specification to “one embodiment”, “anembodiment”, or “a specific embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention and notnecessarily in all embodiments. Thus, respective appearances of thephrases “in one embodiment”, “in an embodiment”, or “in a specificembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics of any specificembodiment of the present invention may be combined in any suitablemanner with one or more other embodiments. It is to be understood thatother variations and modifications of the embodiments of the presentinvention described and illustrated herein are possible in light of theteachings herein and are to be considered as part of the spirit andscope of the present invention.

Embodiments of the invention may be implemented by using a programmedgeneral purpose digital computer, by using application specificintegrated circuits, programmable logic devices, field programmable gatearrays, optical, chemical, biological, quantum or nanoengineeredsystems, components and mechanisms may be used. In general, thefunctions of the present invention can be achieved by any means as isknown in the art. Distributed, or networked systems, components andcircuits can be used. Communication, or transfer, of data may be wired,wireless, or by any other means.

It will also be appreciated that one or more of the elements depicted inthe drawings/figures can also be implemented in a more separated orintegrated manner, or even removed or rendered as inoperable in certaincases, as is useful in accordance with a particular application. It isalso within the spirit and scope of the present invention to implement aprogram or code that can be stored in a machine-readable medium topermit a computer to perform any of the methods described above.

Additionally, any signal arrows in the drawings/Figures should beconsidered only as exemplary, and not limiting, unless otherwisespecifically noted. Furthermore, the term “or” as used herein isgenerally intended to mean “and/or” unless otherwise indicated.Combinations of components or steps will also be considered as beingnoted, where terminology is foreseen as rendering the ability toseparate or combine is unclear.

As used in the description herein and throughout the claims that follow,“a”, “an”, and “the” includes plural references unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise.

The foregoing description of illustrated embodiments of the presentinvention, including what is described in the Abstract, is not intendedto be exhaustive or to limit the invention to the precise formsdisclosed herein. While specific embodiments of, and examples for, theinvention are described herein for illustrative purposes only, variousequivalent modifications are possible within the spirit and scope of thepresent invention, as those skilled in the relevant art will recognizeand appreciate. As indicated, these modifications may be made to thepresent invention in light of the foregoing description of illustratedembodiments of the present invention and are to be included within thespirit and scope of the present invention.

Thus, while the present invention has been described herein withreference to particular embodiments thereof, a latitude of modification,various changes and substitutions are intended in the foregoingdisclosures, and it will be appreciated that in some instances somefeatures of embodiments of the invention will be employed without acorresponding use of other features without departing from the scope andspirit of the invention as set forth. Therefore, many modifications maybe made to adapt a particular situation or material to the essentialscope and spirit of the present invention. It is intended that theinvention not be limited to the particular terms used in followingclaims and/or to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include any and all embodiments and equivalents falling within thescope of the appended claims. Thus, the scope of the invention is to bedetermined solely by the appended claims.

1. A battery cell charging system, including a charger and a controller,for low-temperature (below about zero degrees Celsius) charging alithium ion battery cell, the battery cell charging system comprising: acircuit for charging the battery cell using an adjustable voltagecharging-profile to apply a charging voltage and a charging current tothe battery cell wherein said adjustable voltage charging-profileincludes: a non-low-temperature charging stage for charging the batterycell using a charging profile adapted for battery cell temperaturesabove about zero degrees Celsius; and a low-temperature charging stagewith a variable low-temperature stage charging current that decreasesresponsive to a battery cell temperature falling below zero degreesCelsius.
 2. A battery cell charging system for low-temperature (belowabout zero degrees Celsius) charging a lithium ion battery cell, thebattery cell charging system comprising: a circuit for charging thebattery cell using a variable charge-profile to apply a charging voltageand a charging current to the battery cell wherein said charge-profileis determined responsive to an imaginary R_(Bad) resistance included inseries with an DCR resistance of a DC model for the battery cell whereinsaid R_(Bad) is a function including a component responsive to a batterycell temperature less than about zero degrees Celsius such that V_(cell)is about equal to a maximum battery cell voltage minus charging currenttimes R_(Bad).
 3. A battery cell charging method for low-temperature(below about zero degrees Celsius) charging a lithium ion battery cell,the battery cell charging method comprising: (a) applying, to thebattery cell when a temperature of the battery cell is above about zerodegrees Celsius, a non-low-temperature charging profile; and (b)applying, to the battery cell when a temperature of the battery cell isbelow about zero degrees Celsius, a variable low-temperature stagecharging current that decreases responsive to a battery cell temperaturefalling below zero degrees Celsius.
 4. A battery cell charging methodfor low-temperature (below about zero degrees Celsius) charging alithium ion battery cell, the battery cell charging method comprising:(a) applying, responsive to a charge-profile, a varying charging voltageand a varying charging current to the battery cell wherein saidcharge-profile is determined responsive to an imaginary R_(Bad)resistance included in series with an DCR resistance of a DC model forthe battery cell wherein said R_(Bad) is a function including acomponent responsive to a battery cell temperature less than about zerodegrees Celsius such that V_(cell) is about equal to a maximum batterycell voltage minus charging current times R_(Bad).